The cell biology and biochemistry of Leishmania and Trypanosoma are investigated as models of intra- and extracellular parasitism, respectively. As all interactions between host and parasite occur at the level of the parasite surface membrane (SM), emphasis is placed on: 1) its integrated biochemical and physiological characterization and 2) defining its roles in parasite survival and development. A 76 KDa SM glycoprotein, with putative carboxyl-carrier functions was identified in L. donovani (Ld) promastigotes. A SM L-proline-binding/transport protein containing two subunits of 41 and 34 KDa was identified in Ld. The biosynthetic secretory pathway of soluble-acid phosphatase (S-AcP) was localized in both Ld pro- and amstigotes. S-AcP was purified and the first 30 residues of its N-terminal amino acid sequence determined. Oligonucleotide probes based on this sequence were used to identify and clone the S-AcP gene. The inducible Ld SM 3'- nucleotidase/exo-ribonuclease was isolated, purified and shown to be a 43 kDa mannosylated glycoprotein. Antibodies are being used to screen cDNA-expression libraries to identify and clone its gene. The LD SM 5'-nucleotidase was isolated, purified and shown to be a mannosylated glycoprotein containing two subunits of 72 and 43 kDa. Further, the LD SM glucose transport protein and proton-ATPase were identified and characterized. Based on their homologies with similar proteins in other systems, oligonucleotides of conserved functional regions were synthesized as probes for identifying and cloning the genes for these two leishmanial proteins. The current results provide further functional characterization of parasite SM constituents and these could serve as potential targets for chemotherapy and/or immunodiagnosis/-prophylaxis.